Field of the Invention
This invention relates to a current control apparatus and method for air conditioning apparatus and, more particularly, to a current control apparatus and method which sets limitation of the total current in the cooling operation and heating operation and protects an inverter from excess current by detecting current in the inverter.
Generally, the capacity of an outdoor air conditioning apparatus is determined by the capacity of the compressor. Operating capacity of a compressor is dependent on the speed of rotation of the compressor motor which is dependent on its supply frequency. The supply frequency is controlled by the switching operation of the inverter.
When an inverter is used for controlling a compressor, an air conditioning apparatus accurately detects the total current flowing in the inverter and, if the detected current value is excessive to a predetermined current limitation, the compressor stops so that the current flowing in the inverter is not greater than the predetermined limit value.
The switching elements of an inverter comprise semiconductor elements such as a power transistor. However, the physical characteristics of a semiconductor are sensitive to temperature change. Especially, where there is a wide difference between the outside temperatures during the cooling operation in summer and the heating operation in winter, an air conditioning apparatus with an inverter comprising semiconductor elements has a disadvantage in that, because the quantity of current applied to the compressor varies greatly, the air conditioning apparatus is set on parameter adapted to one of the two seasons malfunctions in other season.
That is to say, if the parameter of the semiconductor is set to operate in the cooling mode, in the heating operation, the quantity of current applied to compressor by the inverter is less so that the rotation speed of the compressor is lower than that required in the heating operation. Therefore, the air conditioning apparatus malfunctions. If the semiconductor is set to operate in the heating mode, to the contrary of the above described case, excessive current flows in the inverter, resulting in the semiconductor element being damaged.
A typical example of the above described air conditioning apparatus is disclosed in Japanese laid open publication No. 58-17997. This apparatus detects excessive current. It not only periodically stops the operation of the compressor but also fixes the rotation frequency of the motor every time an excessive current flows in the inverter. Thus, when the rotation of the inverter motor is reversed by an external power, even if it operates, the direction of its rotation is corrected to the normal rotation direction without halting the operation of the inverter. Thus, the inverter is protected from excessive current.
However, the present invention differs from the above described apparatus in that the semiconductor elements are prevented from breakdown resulting from a large difference in current flowing in the inverter during the heating mode and the cooling mode.
Also, there is an another problem. When a peak current is generated in the inverter, it causes a breakdown of the semiconductor element thereby disturbing normal operation. Accordingly, a conventional apparatus detects peak current by a current transformer or shunt resistor and prevents a malfunction caused by the peak current. However, a system using a current transformer does not detect a minute quantity of current because of the characteristics of transformer. Because current shunted by an resistor is detected by the shunt resistor, a system using a shunt resistor is the same. Therefore, these systems have a problem in that they do not guard against the problem caused by peak current.